The present invention relates to a flexible membraneous weir available as weirs for intake of agricultural water or the like.
A flexible membraneous weir (normally made of rubber and otherwise called rubber weir) laid across a river is erected or collapsed by charging or discharging fluid to or from a bag. Especially in a weir erected or collapsed by charging or discharging air, as the air in the bag is discharged at the time of collapse, a height of the top wall of the bag would not lower uniformly according to lowering of an inner pressure, rather a so-called V-notch phenomenon, in which the top wall deforms in the shape of a letter "V" at one portion or two portions in the river width direction, would occur, and a flow of the river would concentrate into the V-notch location.
The position where the V-notch phenomenon occurs would vary and would not be constant due to the fact that the position of the center of flow varies depending upon the shape of the river at the location where the rubber weir is laid. It also varies depending upon the state of flow and the mode of construction of the rubber weir.
If the V-notch phenomenon would occur at an inadequate location, then in some cases the river bed at downstream side of the V-notch deformation would be scoured, resulting in change of a shape of the river.
Therefore, in a flexible membraneous weir disclosed in U.S. Pat. No. 4,662,783, a river bed surface on the downstream side at the location of the weir, where the V-notch deformation is desired to occur is excavated and a recess is formed there.
In this flexible membraneous weir, at the time of collapse, when the inner pressure has lowered to a certain extent, a membrane portion corresponding to the above-mentioned recess would sink in that recess, and V-notch deformation would occur at that membrane portion.
Observing the process of occurrence of this V-notch deformation, under the condition where the bag of the membraneous weir erects perfectly the bag is held erected upwards by a tension without sinking in the above-mentioned recess, but while the air in the bag is being discharged for the purpose of collapsing, when an inner pressure of the bag has lowered to a certain extent, the downstream side of the portion of the bag corresponding to the recess would sink in the recess, and as pulled by this sunk bag portion, the upstream side of the same bag portion would become sunk, and thereby a V-notch deformation is formed.
As described above, since a V-notch deformation is formed on the upstream side indirectly as a result of sinking of the downstream side of the bag, there exists a possibility that a V-notch deformation may occur earlier at an unpredictable location due to any other important factor.
In addition, in order to excavate the river bed surface, a construction expense becomes large and a construction term is also elongated.
Frequently, such a flexible membraneous weir is used for intake of a river water as agricultural water for example. In this case, the flexible membraneous weir is laid across a river at downstrae side of a water intake which is provided in either bank of the river and branches off from that river. When the weir is erected, water level of the river at upstream of the weir is raised to allow a sufficient quantity of the river water to flow into the water intake.
However, earth and sand are apt to accumulate in the portion of the water intake to reduce the intake flow rate through the water intake.
The present invention has been worked out in view of the above-mentioned circumstance in the prior art.